Herpes simplex virus (HSV) type 1 and 2 are serologically related but show major differences in the percent G plus C, Tm and buoyant densities. By hybridization HSV-1 and HSV-2 DNA are approximately 50% homologous. A sensitive hybridization procedure has been developed to map specific regions of homology and to physically relate the linear genomic maps of HSV-1 and HSV-2.